Prothallus

“ The prothallus is exceedingly small, being little more than a group of cells forming an antheridium. Should there be sufficient moisture about, the spermatozoids which are produced by the antheridium swim towards the archegonia in any female prothallus which may be near. Fertilization then takes place, and the final outcome is the young plant. Now and again in certain species it is seen that the megaspores develop to such an extent within the sporangium that fertilization [21] takes place, and even an embryo or young plant may be formed. ”

“ The new plant is, however, not a horsetail, but a small filmy prothallus, somewhat like the prothallus of a fern. Some of the prothalli produce male cells, while others give rise to female cells. When the minute male cells are ripe, they are set free, and are able, by means of fine, lashing threads, to swim towards the female cells of a neighbouring prothallus, through a drop of dew or rain. The two cells fuse together and give rise to a little embryo, which in due course grows up into a new plant—an ordinary horsetail with stems and branches and roots like its grandparent. ”

“ The spores of the Horsetails are not long in developing after they have settled in a damp situation. The actual forms which the prothalli take are often very irregular. That of the male prothallus is usually rather small; on the other hand, the female prothallus is sometimes large, and may have complicated branchings. As in the case of the other Vascular Cryptogams which have been considered, spermatozoids are produced in the antheridia. These are very active, and travel through the agency of water to the archegonia on the female prothallus. ”

“ The sperm-cells escape from the antheridium and in the water that collects on the prothallus are carried to the archegonium, where fertilization of the egg takes place. From the fertilized egg-cell a plant grows, becoming a “fern.” In most cases the prothallus soon dies. The prothallus is the gametophyte (from Greek, signifying the fertilized plant) .
The fern plant, arising from the fertilized egg in the archegonium, becomes a perennial plant, each year producing spores from its fronds (called the sporophyte) ”

“ After a time, however, the cells are produced transversely as well as longitudinally, resulting in the formation of a flattened leaf-like expansion (Fig. 125, d) , a cellular structure, the so-called Prothallus, which can scarcely be distinguished from a young Marchantia. In this Prothallus are developed Archegonia and Antheridia: the union of the embryo-cell of the Archegonia and the spiral filament of the Antheridia gives rise to the new Fern, which may be seen growing out of the Prothallus (Fig. 126) , which soon passes away. ”

“ In the genus Selaginella the female spore produces a small prothallus, as shown at 1, Fig. 14. The portion above d d, in this cross-section of the spore, is the prothallium, and at e e are two embryo plants. At 2 is a young archegonium not opened; 3 shows one further advanced, with the fertilized germ-cell divided. A is a male spore, showing the cell-division ”

“ The prothallus grows in size proportionately more rapidly than the other organs, and this is accomplished largely at the expense of the nucellus, which gradually becomes thinner throughput, and is finally, at the time fecundation occurs, found to be compressed to a very thin membrane at the apex, and below on the sides and base has largely split up into very thin shreds, seldom being found as an unbroken membrane throughout. ”

“ For this spore, though so small as to be visible only by microscopic aid, had a previous history, which may be traced without difficulty. It was generated with hundreds [Pg 118] more, in one of many capsules, which, were crowded together, beneath the oval bit of membrane, that covered one of the brown spots (sori) , which were developed in the under surface of the fronds of an earlier Lady-fern. That earlier individual had in turn passed through the same stages of sporal frond, prothallus, spore, theca, sorus, frond, prothallus, spore, theca, sorus, frond, prothallus, &c.—ad infinitum. ”

“ The spindle, from the crescent shape of the second prothallial cell, assumes a position at an angle to the major axis of the prothallus, the lower nucleus and end of the spindle being crowded to one side by the position of the first prothallial cell, while the upper nucleus occupies a central position in the upper half of the cell which, when the new wall is formed, will become the central cell. The lower nucleus, which becomes the nucleus of the stalk cell, is already in this early stage noticeably smaller than the upper nucleus. ”

“ They thus assist in the opening of the sporangium, which takes place by a slit on its inner face. Further, several spores will be likely to germinate together owing to their elaters becoming entangled; a fact of some importance, since the antheridia and archegonia, though occurring sometimes on the same prothallus, are more often borne on separate individuals. The prothalli contain abundant chlorophyll, and are dorsiventral. Those that bear the antheridia are the smaller, and are either filamentous, or flattened, and irregularly lobed. The antheridia are deeply sunk in the tissue ”

“ In the development of the sporangium the sporogenous tissue is derived from a number of superficial cells by divisions parallel to the surface. The tapetum is derived from the layer of cells surrounding the sporogenous group. Short trabeculae of sterile tissue have been found to project into the cavity of the sporangium of some species. The spores, when liberated by the dehiscence of the sporangium, give rise to the prothallus, which is now, owing mainly to the investigations of Treub and Bruchmann, known in a number of tropical and temperate species. ”

“ Further than this the spore alone has no power to go, and the prothallium is not truly the germ of the future plant. True germs, needing fertilization, are produced upon it, and also the means whereby they can be fertilized. These can be distinguished only by use of the higher microscopic powers. If a portion of the prothallium is examined, it will be found studded with little bladders, containing round semi-transparent bodies of a greenish hue. ”

“ Thus, proto and heteroproteose are primary bodies formed directly from the initial product syntonin by the further action of the ferment. In the same sense, deutero­proteose is a secondary proteose, being formed by the further hydration of the primary body. Lastly, peptones, the final products of pepsin-proteolysis, are the result of the hydration and possible cleavage of deutero­proteoses. Further, in almost every gastric digestion there is also formed a small amount of antialbumid, a product insoluble in dilute hydrochloric acid and which consequently appears as an insoluble residue. ”

“ The spores, which are capable of living only a short time, grow into aerial green prothallia, 1–2 cm. in length; these have the form of irregularly and more or less deeply lobed structures. On the larger and more deeply lobed prothallia the archegonia or female reproductive organs are borne, and the smaller or male prothallia bear the antheridia. On the fertilisation of an egg-cell, the Equisetum plant is gradually developed. For a short time parasitic on the female prothallium or gametophyte, the young plant soon takes root in the ground and becomes completely independent. ”

“ Still, there are other differences between natural digestion and artificial proteolysis than those connected with the possible absorption of the more diffusible products of digestion. Thus, in the living stomach there is an ever-increasing secretion of hydrochloric acid, and perhaps also of pepsin, more or less proportional to the extent of proteolysis. On this point Brandl’s experiments again give us some light. Thus, it was found that the introduction of an aqueous solution of peptone into the empty stomach led to the secretion of an acid fluid containing on an average 0.24 per cent. ”

“ The proteolytic power was proved to be due to a specific ferment or enzyme, now universally called trypsin, which digests proteid foods to the best advantage in the presence of sodium carbonate. Digestive proteolysis in the human body was thus shown to be due mainly to the presence of two distinct enzymes, the one active in an acid fluid, the gastric juice, the other in an alkaline-reacting fluid, the pancreatic juice, but both endowed with the power of digesting all varieties of proteid foods, with the formation of a large number of more or less closely related products. ”

“ At first there is a marked and rapid progression; the proteid undergoing proteolysis is rapidly dissolved, and both proteoses and peptones may be detected in abundance. But if we continue to watch the changing relations of primary and secondary proteoses and peptones, we find that progression soon ceases to be rapid, and eventually travels onward at a snail’s pace. Thus, in one experiment with coagulated egg-albumin, there was found at the end of forty-eight hours’ digestion with pepsin-hydrochloric acid, only thirty-seven per cent. ”

“ Peptones, as we have seen, are possessed of a higher endosmotic equivalent than the proteoses, but we need to keep continually in mind the possibility that the selective power of the epithelial cells of the intestinal mucosa may lead to as rapid transference of the proteoses as of the more diffusible peptones. It is not to be understood by this, however, that diffusibility is of no consequence in determining the rate of absorption. Surely, everything else being equal, the more diffusible the substance the more rapid will be its passage from the intestine into the blood-current. ”

“ This body can be reconverted into heteroproteose, in part at least, by solution in dilute acid, or alkali, and reprecipi­tation by neutralization.
As a class, the proteoses are characterized by far readier solubility in water than native proteids, by a far greater degree of diffusibility, by non-coagulability by heat and by alcohol, although precipitable by the latter agent. Further, nearly all proteose precipitates are exceedingly sensitive toward heat, tending to dissolve as the fluid is warmed and reappearing as the solution cools. ”

“ These two experiments thus give striking proof that free acid is not absolutely essential for pepsin-proteolysis. Digestion is, to be sure, more rapid and complete when free hydrochloric acid is present, but proteolysis is still possible, and even vigorous, when there is a marked deficiency of free acid. Further, as we have seen, proteolysis may proceed to a certain extent even though the amount of acid available is not sufficient to combine with more than half the proteid matter present. ”

“ The Protista that live in water undergo a death by "diffluence" or "granular disintegration" on being wounded, crushed, or sometimes after an excessive electric stimulation, or contact with alkalies or with acids too weak to coagulate them. In this process the protoplasm breaks up from the surface inwards into a mass of granules, the majority of which themselves finally dissolve. If the injury be a local rupture of the external pellicle or {15}cuticle, a vacuole forms at the point, grows and distends the overlying cytoplasm, which finally ruptures: the walls of the vacuole disintegrate ”

“ Further, the recent experiments of Huber200 have given us quantitative data on the rate of absorption of fluid egg-albumin when introduced into the large intestine in the form of a clyster, showing that even fairly large amounts of a natural proteid may be absorbed without undergoing proteolysis if mixed with a neutral salt, like sodium chloride. To be sure, the rate of absorption is greatly increased when the albumin has been peptonized, but still absorption of the native proteid is possible without the agency of proteolytic enzymes. ”

“ In pancreatic digestion, proteids are exposed to the action of an enzyme of much greater power than pepsin, one endowed with a far greater range of activity, and consequently proteolysis as it occurs in the small intestine becomes a broader and more complicated process. As you well know, the ferment trypsin manifests its power not only in a more rapid transformation of insoluble proteids into soluble and diffusible products, but there is a diversity in the character of the many products formed which testifies to the profound alterations this ferment is capable of producing. ”

“ Thus, when it was suggested that ammonia, NH3, might be formed under the influence of trypsin, it was not considered at all improbable, for in the hydrolytic decomposition of proteids by boiling dilute acid, as well as by baryta water, it had long been known as a prominent product. Obviously, in trypsin-proteolysis, the one thing to be guarded against in proving the formation of ammonia is the contaminating influence of bacteria. ”

“ It is thus plain that these enzymes, capable though they are of accomplishing great tasks, are nevertheless exceedingly unstable and prone to lose their proteolytic power under the slightest provocation. When, however, they are surrounded by their natural environment, the acid or alkali of the respective secretion, together with salts and proteids, they then appear more stable; their natural lability becomes for the time being transformed into semi-stability, and the temperature, for example, at which they lose their peculiar power, is raised ten degrees or more. ”